In order to facilitate their handling, vehicles are sometimes provided, where appropriate, with automatic gearboxes. The term automatic gearboxes relates to all types of gearboxes in which gears are selected without the direct intervention of an operator or driver. An example of an automatic gearbox in which gears are selected via an electronic control unit is given in EP 155043. Where appropriate, automatic gearboxes can be set to an operating mode in which the operator or driver selects gears by activating a gear selector. An example of a gear selection device is given in WO 01/55622. The invention can also be used in a type of gearbox in which the device described in WO 01/55622 is used, in which gears are selected without the direct intervention of an operator or driver.
In known electronically controlled automatic gearboxes, downshifting to a lower gear is controlled in that a control element receives information on the current rotational speed of the gearbox or alternatively the current rotational speed of an engine coupled to the gearbox. Downshifting is done in order to ensure that sufficient torque is available to drive the gearbox. FIG. 1 shows a torque curve giving the torque delivered as a function of the speed of a diesel internal combustion engine. The torque curve shows a relatively steep flank from the idling speed, after which the torque levels out around an operating range and then falls sharply. In order to achieve good driveability in a vehicle having an internal combustion engine, a signal for downshifting to a lower gear is emitted when the internal combustion engine is in an operating condition corresponding to an engine speed situated approximately midway along the steep flank. For conventional diesel engines intended for heavy trucks the gearbox control unit is designed with a higher transmission ratio of up to approximately 800-1200 rpm, depending, for example, on load, gradient and accelerator pedal position. A common idling speed for internal combustion engines intended for heavy trucks is approximately 600 rpm.
Gearboxes for heavy vehicles are normally equipped with a large number of gear positions. A normal number of gears for a vehicle in forward drive may be 12 to 18 gears. This means that the difference in transmission ratio between two gears is relatively slight, and therefore a large number of gears will be encountered when running at varying speed or varying torque. This is normally positive when a heavily laden vehicle can accelerate freely from stationary to the desired running speed. On the other hand, gearshifts are experienced as problematical in driving situations in which a driver has temporarily released the throttle before accelerating away immediately thereafter, as when a vehicle slows down in cornering or taking a roundabout, for example. Another example of operating interference due to frequent gearshifts is when a vehicle has to be driven in a line of traffic, in which a sequence of gearshifts under torque occurs despite the fact that the driver primarily wishes to maintain and adjust the speed of his vehicle to the speed of the line of traffic. Unwanted gearshifts generally cause characteristics in the vehicle that are difficult to control, firstly since the drive torque to the wheels of the vehicle varies with the gear ratio in the gearbox, and secondly because the gearshift sequence takes a relatively long time during which the working of the drivetrain is interrupted. The overall result is a drive torque that is difficult to control, to the detriment of the vehicle handling characteristics.